1. Field of the Invention
This invention pertains generally to a solar concentrator and more particularly to a low power solar concentrator having thin film reflective panels for use on a spacecraft.
2. Description of the Related Art
Most satellites generate electric power in space by exposing solar panels--flat panels of photovoltaic cells--to the Sun. One way to increase the power from a solar panel is to use reflectors to direct additional sunlight onto the solar panel. For example, doubling the amount of sunlight onto the panel would approximately double the electric power output of the panel. (This power increase is always less than directly proportional to the increase in incident sunlight because the increased sunlight raises the temperature of the cells which in turn decreases their efficiency.) The measure of effectiveness is the increase in specific power, that is, the power output of the augmented solar arrays, divided by the mass of the solar panels plus reflectors. This ratio should be substantially greater than the power of the unaugmented panel divided by its own mass. Therefore, it is evident that the added reflectors should be extremely light.
Solar concentrators utilized on spacecraft can be divided into three categories depending on their concentration ratio, that is, the ratio of area of sunlight gathered to the area of photovoltaic cells on which the sunlight is concentrated. First a parabolic reflector is used in high concentration systems (those with a concentration of approximately 50:1 or more). Secondly, Fresnel lenses for use in medium concentrator systems (concentrations in the range of 10:1 to 25:1), and thirdly, the trough type described in this invention for use in low power concentrators with a ratio less than 3:1. Because of the heat generated, the first two types require a special cooling means for the photovoltaic cells.
In normal operation, the solar panels are capable of being folded and stowed against the sides of the spacecraft during launch to minimize the spacecraft volume and to protect the fragile panel from launch loads and vibrations. Once in orbit, the panels and reflectors are unfolded, as in the third case, to form a trough configuration.
Further, the reflector must be flat in order to provide an even distribution of sunlight across the solar panel since any irregularities decrease the power output of the panel. In the prior art, flatness was achieved by using rigid reflectors which resulted in excessive mass for the system. Further, in the case of the trough reflector configuration, the prior art is limited in that the width of the rigid reflector is limited to, at most, the width of the solar panel because the rigid reflector panel must fold against the photovoltaic panel during launch. This limits the concentration ratio. Further, the solar panel cannot produce electric power until it deploys after reaching its final orbit if the reflector panel covers the outside of the stowed solar panel. Higher concentrations can be achieved by having rigid reflectors with double folds, but this increases the mass and complexity of the system.